title: The minimal number of antigen-major histocompatibility complex class II complexes required for activation of naive and primed T cells.
abstract: Although previous studies have shown that 50-200 antigen-major histocompatibility complex complexes (Ag-MHC) are sufficient to stimulate significant secretion of interleukin (IL)-2 from MHC class II-restricted T cell hybridomas, there have been no studies of this nature on more physiologically relevant T cell populations. In this study we have analyzed the ligand requirements for stimulation of responses from naive and previously primed T cells derived from T cell receptor (TCR)-transgenic animals whose TCR is specific for the pigeon cytochrome c (PCC) 88-104 peptide presented by I-Ek. Primed T cells were as sensitive as the previously reported T cell hybridomas, requiring about 100 Ag-MHC complexes to synthesize readily detectable quantities of IL-2, whereas naive T cells required 15 times more ligand to produce equivalent quantities of IL-2. Similarly, primed T cells required about 40 Ag-MHC complexes to produce a significant proliferative response, whereas naive T cells required about 400 complexes. In contrast to these results, naive and primed T cells showed similar ligand requirements when early events in the T cell activation pathway were analyzed; i.e. TCR down-modulation, CD69 and CD25 expression, and blast transformation. A further analysis of IL-2 and IL-2R expression indicated: 1) The first synthesis of IL-2 was detected at the same ligand concentration in both primed and naive T cells, but primed T cells made much more IL-2 as the ligand concentrations increased; 2) primed T cells expressed about fivefold more IL-2 receptor (R) than naive T cells, despite the fact that the antigen dose-response curves with respect to the percentage of cells expressing IL-2R were identical. These results suggest that naive and primed T cells have the same threshold with respect to the number of Ag-MHC complexes required to initiate T cell activation, but that due to the inefficient expression of IL-2 and IL-2R, engagement of more complexes is needed to enable naive T cells to synthesize the necessary amounts of these two molecules to allow T cells to go through a complete cycle of replication.
authors.affiliation: La Jolla Institute for Allergy and Immunology, San Diego, CA 92121, USA.
authors.forename: K
authors.initials: K
authors.lastname: Kimachi
authors.forename: M
authors.initials: M
authors.lastname: Croft
authors.forename: H M
authors.initials: HM
authors.lastname: Grey
mesh_terms: D000818:Animals; D017951:Antigen Presentation; D000949:Histocompatibility Antigens Class II / Q000276:immunology*; D006825:Hybridomas / Q000276:immunology*; D008213:Lymphocyte Activation; D051379:Mice; D008822:Mice, Transgenic; D010455:Peptides / Q000276:immunology; D011940:Receptor Aggregation; D013601:T-Lymphocytes / Q000276:immunology*
chemical_list: D000949:Histocompatibility Antigens Class II; D010455:Peptides